Ordnance fuze firing circuit



Feb. 17, 1959 HIGH IMPEDANCE SOURCE VOLTAGE AT THYRATRON GRID H. F. GIBSON ORDNANCE FUZE FIRING CIRCUIT Filed Jan. 9; 1958 cmmeme souncs FILAMENT lvoLrAGE SOURCE HAROLD E maso nfted States Patent 2,813,619, oRnNANcE FUzE rnuNG CIRCUIT Harold F. Gibson, Silver SpringQMd., assignor to the United States of America as represented by the Secretary of the Army Application January 9, 1958, Serial No. 708,055

(Granted under Title 35, U. S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to me of any royalty thereon.

This invention relates to a novel ordnance fuze tiring circuit. Y

An object of this invention is to provide a novelV ordnance fuze tiring circuit adapted to be triggered by a signal from either a high impedance or a low impedance signal source.

Another object is to operate said ordnance fuze firing circuit utilizing a charged capacitor as the only voltage source except for heater supply.

A further object of this invention is to incorporate. 1n

said firing circuit, novel means for sensing the apphed input signal, maintaining the circuit safe for a predetermined time after energization, and providing a holdlng bias which prevents tiring unless the input signal has a predetermined minimum amplitude.

Still another object is to additionally incorporate 1n said firing circuit, means for causing. the circuit to self fire after a predetermined time in the event that an mput signal has failed to cause tiring within this time.

An additional object of this invention is to provide these objects in a simple, economical, and reliable circuit.

The above objects are accomplishedby the novel combination of an electrometer-connected pentode with a thyratron and a detonator.

The specific nature of the invention, as-well as other objects, uses, and advantages` thereof, will clearly appear from the following description and from the accompanying drawing, in which: 1

Figure 1 is a circuit diagram of a high impedance ordnance fuze tiring circuit in accordance with the invention.

Figure 1A is a partial circuit diagram showing a low input impedance modification of Figure 1.

Figure 2 is a graph showing the Voltage on the thyratron grid during circuit operation. Y Y In Figure `1, a thyratron 12 has a plate 14, a grid 16, and a directly heated cathode 18 connected to circuit ground 19. A capacitor 20 is connected between the plate 14 and ground 19 through a detonator 15. In the grid circuit of the thyratron 12 between the grid 16 and ground 19 are connected resistors 26 and 28 in series. A charging capacitor 22 has one end connected to the junction 31 between the resistors 26 and 28 and the other end connected to the movable ann 34 of a single-pole, double-throw switch 33. The terminal 35 vof theswitch 33 is connected to a charging source and the terminal 36 is connected to Vthe junction between the detonator and "ice the capacitor 20. When the switch 33 is in the position shown in the drawing, the charging capacitor 22 charges to a predetermined voltage.

It is to be understood that the location of the detonator 15 in Figure 1 is for purposes of simplicityand clarity and forms no part of the invention. Actually, it is preferable for safety reasons to locate a fuze detonator with one end connected to circuit ground. This may be accomplished in a variety of well known ways. In Figure l for example, the detonator 15 could be connected in series with the capacitor 20 with one end connected to circuit ground 19. An inductance placed across the detonator 15 may be used to serve as a short circuit to charging currents, and as a high inductance to the rela-` tively fast thyratron tiring pulse so as to permit initiation of the detonator 15.

A pentode 42 is connected with its control grid 47, suppressor grid 45, and grounded directly heated cathode 48 connected together. The screen grid 46 is left oating and provides a high impedance input for the signal from the high impedance source. If so desired a small capacitor 61v (about l0 micro-microfarads for example) may be connected to grid 46 and the circuit used with a low impedance source, as shown in Figure 1A. A plate resistor 51 is connected between the terminal 36 and the plate 44 of the pentode 42, and a coupling capacitor 53 is connected between the plate 44 of the pentode 42 and the grid 16 of the thyratron 12. A filament source applies filament voltage to the parallel-connected, directly heated cathodes 18 and 48.

The operation of the circuitin Figure 1 may be explained with reference to Figure 2 which shows the voltage on the thyratron grid 16 during circuit operation. In response to some desired event, such as arming, the movable arm 34 of switch 33 is caused to switch from terminal 35 to terminal 36 thereby energizing the circuit. This applies the voltage to which charging capacitor 22 has been charged across terminal 36 and the junction 31 of resistors 26 and 2S. The capacitors 20 and 53 will thus begin to charge, the capacitor 20 being charged through the resistor 28, and the capacitor 53 being charged through the resistors 51 and 26 in series. The charging capacitor 22 is preferably chosen to be considerably larger than either of capacitors 2t) or 53.

The charging of the capacittor 20 through resistor 28 causes a large exponentially decaying negative voltage to appear at the thyratron grid 16 as shown by A in Figure 2. The thyratron 12 is of the type which triggers at some small negative grid voltage. The thyratron triggering voltage throughout circuit operation is shown by B. The large decaying negative Voltage A initially prevents triggering of the thyratron 15 thus maintaining the circuit insensitive for some predetermined time. The resistor 26 is made considerably smaller than the resistor 51 so that the Voltage appearing across the resistor 26 as the capacitor 53 charges will be small as compared to the voltage A across resistor 28. When the voltage'between the plate 44 and cathode 48 of pentode 42 builds up to about a few volts, the electrometer-connected pentode 42 begirls to conduct causing a current I to iiow through the resistor 28. This point is indicated by C inFigure 2. The electrometer connection of the pentode with floating grid 46 provides a high efective impedance for the pentode 42.

The flow of current I through resistor 28 causes the thyratron grid voltage to deviate from the path A1 it would have followed in the absence of current I, and follow the path E as shown in Figure 2. To prevent premature triggering of the thyratron 12, it will be noted that the circuit must be adjusted iirst, so that the pentode 12 starts conducting before the decaying negative voltage A---A1 falls below the thyratron triggering voltage B. It will be understood that this adjustment may be accomplished by proper choice of the charging time constants of capacitors and 53. Secondly, the circuit must be adjusted so that the values of the current I and the resistor 28 will be such as to cause the thyratron voltage to follow a path similar to E.

As shown in Figure 2, an essentially steady-state condition will then be reached with a slowly decaying holding bias voltage H appearing on the thyratron grid 16 caused by current I flowing through resistor 28. One way of .obtaining this steady-state condition is to make the resistor 51 considerably larger than the effective resistance of the pentode 42. Thus, the rate of decay of the holding bias voltage H will be substantially determined by the rate of decay of the voltage on the capacitor 22.

When a negative input signal appears at the floating grid 46, two effects occur which act to reduce the negative thyratron grid voltage. First, a positive signal is applied to the thyratron grid 16 through the coupling capacitor 53 as illustrated by D in Figure 2. Second, the current I is reduced so that the holding bias voltage acrossresistor 28 decays (with the same time constant as did A) to a lower value as illustrated by K. The' thyratron conduction pulse is represented by D1. It should be noted that the magnitude of the signal passed through the coupling capacitor 53 as illustrated by D in Figure 2 is dependent upon the ratio of the resistors 26 and 28 to the plate resistor 51. Since the resistor 51 is relatively large as described previously, the actual signal obtained from this effect will not be very much larger than the input signal, but will be no worse than is obtained with conventional high impedance pentodes which often use a high plate resistor such as 51. However, because of the presence of the second effect, the actual sensitivity achieved is considerably better than that which is obtained with the conventional pentode used as a high impedance amplifier.

The overall sensitivity ofthe circuit of Figure l is relatively constant throughout curve H and part of curve G as shown in Figure l. This is shown by the fact that thyratron'triggering curve B is relatively parallel to curve H, and also to curve G over a wide range. This occurs because although the holding bias voltage is decaying as shown by H and G, the Voltage on the capacitor 20, which is the voltage applied to the plate circuit of the thyratron 12, is also decaying causing the thyratron triggering voltage curve B to decay also. Since the plate voltage applied to the pentode 42 is likewise decaying, additional compensation will be afforded by the resulting decay of pentode sensitivity. In the event that no input signal of sufficient amplitude to cause thyratron triggering appears, the holding bias will slowly decay as shown by G. Towards the end of G it can be seen that the overall sensitivity of the circuit will increase rapidly. When the holding bias voltage curve G intersects the triggering voltage curve B as shown at F, the thyratron 12 will self-tire producing a thyratron conduction pulse F1 which res the detonator 1S.

It should `be noted that the above-described features are obtained in a simple and economical circuit utilizing a charged capacitor 22 as the only voltage source (not counting the lament voltage source).

The operation of the embodiment of Figure l may be summarized as follows. In response to some event such as arming movable arm 34 of switch 33 switches from terminal 35 to terminal 36 causing capacitor 22 to begin to charge capacitors 20 and 53. The charging of capacitor 20 causes a large exponentially decaying negative voltage A to appear at the thyratron grid 16 preventing triggering of the thyratron 14. While the capacitor 20 is charging, the capacitor 53 is also charging and ncreasing the voltage across the plate 44 and cathode 48 of pentode 42. When this plate to cathode voltage builds up to a few volts, the pentode 42 conducts (point C) causing a current I to pass through the resistor 28 establishing a slowly decaying holding bias voltage H on the thyratron grid 16. When a negative voltage now appears at the pentode grid 46 (point D), resulting from target proximity, for example, it simultaneously causes: (l) a positive signal to be applied to the thyratron grid 16 through the capacitor 53, and (2) a reduction in the current I. These two effects combine to produce a triggering signal at the thyratron grid 16 which results in the capacitor 20 rapidly discharging through the thyratron 14 to fire the detonator 15. In the event that a sufcient input signal is not applied to the pentode grid 46 within a predetermined time (determined by the circuit time constants), the holding bias slowly decays to a value which causes the thyratron 12 to self-tire (point F).

Typical values for the components of the circuit of Figure l are as follows: charging capacitor 22, 0.5 microfarad; capacitor v20, 0.1 microfarad; capacitor 53, 0.03 microfarad; resistors 28 and 26, 10 megohms; and resistor 51, 500 megohms. A conventional pentode utilized in an electrometer connection for the pentode 4Z has a steady-state effective resistance of about 10 megohms, and an input impedance of about 1010 ohms. The thyratron 12 used triggers` somewhere between 0 and -2 volts on its grid, depending upon its plate voltage. With the charged capacitor 22 initially charged to about 240 volts, the circuit fires on an input signal of about l volt during the period l0 to 100 seconds after closing the switch 33. Thereafter the sensitivity increases until at about 10 minutes it becomes unstable and self-tires.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claim.

I claim as my invention:

An ordnance fuze firing circuit comprising in combination the following elements: a thyratron having a plate, grid, and cathode, said cathode being connected to circuit ground, a detonator adapted to be tired when said thyratron triggers, a iirsty capacitor effectively connected between the thyratron plate and circuit ground, rst and second resistors, one end of said first resistor 'being connected to said grid and one end of said second resistor being connected to circuit ground, the other ends of said resistors being connected together to form a junction, a single-pole, double throw switch having a movable arm and first and second terminals between which said movable arm may be switched, said movable arm being initially connected to said rst terminal, said first terminal being connected to a charging source and said second terminal being connected to the junction 'between said detonator and said first capacitor, a second capacitor connectf ed between said movable arm and the junction between said tirst and second resistors, said capacitor being initially charged to a predetermined voltage by said charging source, a pentode having a plate, suppressor grid, screen grid, control grid and cathode, the pentode suppressor grid, control grid and cathode being connected to circuit ground, said screen grid serving as an input, a third resistor connected between said second terminal and the plate of saidpentode, and a third capacitor serving as a coupling capacitor connected between the plate of said pentode and the grid of said thyratron; the elements of said circuit being constructed and arranged so that when said movable arm is switched to saidsecond terminal, the charging of said rst capacitor causes a decaying negative voltage to appear at the grid of said thyratron preventing triggering of said thyratron for a predetermined time, the charging of said third capacitor causing said pentode to conduct and pass a current through said second resistor to produce an essentially steady-state holding bias, said holding bias being produced before said decaying negative voltage falls below the triggering voltage of said thy ratron, said holding bias slowly decaying at a rate determined by the discharge rate of said second capacitor to self-fire said thyratron in the event that a suicient input signal to cause thyratron triggering does not appear within a predetermined time.

References Cited in the tile of this patent UNITED STATES PATENTS OTHER REFERENCES Proximity Fuzes for Artillery, by Harner Selvidge. Electronics Magazine. February 1956. Pages 104 through 109 are pertinent. Classied lOl-70.2 P. 

